Strange.
Are you able to load pages like
and it is only when you try to upload an image that it fails?
I can't think of why that page would be different, except maybe that it is a POST, while the rest would be GET requests. Maybe there is a misconfigured cache or proxy between you and the internet? But then I wouldn't expect https to work at all...?
Is there any way you can post your image someplace accessible on the web, and then submit a URL to
nova.astrometry.net, rather than trying to upload the whole file?
How big are your images?
Sorry for the trouble... I'm not sure what to suggest, because the
nova.astrometry.net site really isn't doing anything fancy or unusual!
cheers,
dustin